The design of reliable, efficient steam turbines requires the application of many diverse areas of technology. There are many competing design and material requirements that must be thoroughly evaluated, so that optimum trade-offs can be achieved. The overriding objective in all steam turbine design activities is to produce steam turbine designs which minimize the life cycle cost of ownership.
The total cost of ownership of a steam turbine generator can be considered to be made up of two components. The first of these is the initial cost or purchase price for acquiring and installing the equipment. The second component is the subsequent operating costs for fuel, operation and maintenance, and unplanned outages.
Of primary significance is that factors such as reliability, efficiency, availability, maintainability and operability, which are functions of the design and construction of the equipment, will affect the operating and maintenance costs of the user. At the same time, marketing pressures require that to be competitive, the manufacturing cycle, installation cycle, and life cycle costs of turbines be reduced without impact on performance and quality.
Opposed flow high pressure/intermediate pressure steam turbines are typically constructed so that the high and intermediate pressure sections are contained within a single outer casing. However, separate inner shells are employed for the high and intermediate sections (as will be explained in more detail below, in connection with FIG. 1). This arrangement has resulted in significant manufacturing and shipping cycles and related costs.
With reference now to FIG. 1, an opposed flow high pressure/intermediate pressure steam turbine 10 in accordance with conventional practice is illustrated. This turbine includes a high pressure section designated HP, and an intermediate pressure section designated IP. From left to right, this conventional turbine includes a thrust beating 12 and a journal bearing 14 along with steam packing 16. The turbine also includes an outer high pressure shell 18, nozzle diaphragms 20, a high pressure exhaust 22, bucket blades 24, and an inner high pressure shell 26. High pressure steam inlets are shown at 28, with a first stage nozzle box shown at 30. Additional steam packing is provided at 32 to separate the high pressure section from the intermediate pressure section.
The intermediate pressure section IP has a separate inner shell 34 adjacent a cross-over pipe 36, and also includes nozzle diaphragms 38, and bucket blades 40. Additional steam packing is provided at 42, and a suitable journal bearing is shown at 44. A reheat or intermediate pressure inlet is shown at 46.
A single high pressure rotor 48 is also shown, extending between the high pressure and intermediate sections HP and IP of the turbine 10.
In the illustrated design, the high pressure (HP) and intermediate pressure (IP) sections are designed so that the various stages are all contained within the single outer casing or shell 18, in an opposed flow arrangement. In such arrangements, the HP section is combined with a single flow IP section within the same beating span as defined by journals 14 and 44. This results in a significantly more compact turbine and stage arrangement than that of a unit with high pressure and intermediate pressure or reheat sections in separate bearing spans.
High pressure steam enters the center of the section via inlet 28 and flows in one direction, i.e., to the left in FIG. 1, while steam reheated to a similar temperature also enters near the center via intermediate section inlet 46 and flows in the opposite direction. This opposed flow arrangement confines the highest temperature steam to a single central location and results in an even temperature gradient from the center towards the ends, with the coolest steam adjacent to the end packings 16, 42 and bearings 14, 44.
The steam turbine of the type described hereinabove, is also designed for partial arc admission to improve part load efficiency. With partial arc admission, the first stage nozzles are divided into separate nozzle chambers or arcs (typically four for larger units with nozzle boxes and six for smaller units), and each arc is independently supplied with steam by its own control valve.